自然界探秘PDF电子书下载

Contents 1

1．Why is the solar system the way it is？ 1

2．What keeps the sun shining？ 3

3．Why do sunsets look red？ 5

4．What is the earth made of 7

5．Why are eclipses so rare？ 9

6．What is a constellation？ 11

7．What is the brightest star？ 13

8．What is a comet？ 15

9．Why do astronomers think there may be life on Mars？ 17

10．Why are there different kinds of clouds？ 19

11．What is a nebula？ 22

12．Why is the ocean salty？ 24

13．Which is the deepest ocean？ 26

14．What causes the waves？ 28

15．What are tidal waves？ 30

16．What is a whirlpool？ 32

17．What is the Gulf Stream？ 34

18．Why does water flow out of a spring？ 36

19．How were the mountains made？ 38

20．Why are there so many different kinds of rocks？ 40

22．What are fossils？ 44

23．How are icebergs formed？ 46

21．How are caves formed？ 48

24．When did the Ice Age end？ 48

25．Why do we still have glaciers today？ 50

26．Why do volcanoes appear only in certain places？ 52

27．Where do most earthquakes occur？ 55

28．How does the weatherman know what to say? 57

29．Why do we have different seasons？ 59

30．What is the jet stream？ 61

31．What is humidity？ 63

32．What is a storm？ 65

33．Why does thunder follow lightning？ 67

34．Why is it hot at the Equator？ 70

35．Is the South Pole as cold as the North Pole？ 72

36．Why is a desert dry？ 74

37．What is smoke？ 77

38．What is smog？ 79

39．What is gas？ 81

40．What is helium？ 84

41．What is atomic energy？ 86

42．What is radium？ 88

43．What is radioactivity？ 90

44．What are X-rays？ 92

45．What are cosmic rays？ 95

46．What is the sound barrier？ 97

47．What causes an echo？ 100

48．What is oxygen？ 102

49．What is water？ 104

50．How is soil formed？ 106

51．Why are there different kinds of sand？ 109

52．How is coal formed？ 111

53．What is a mineral？ 114

54．What is silver？ 116

55．What is quicksilver？ 119

56 What is quartz？ 121

57．What is mica？ 123

58．What is a gem？ 125

59．Do the Northern Lights appear in the South？ 128

60．Where did the stars come from？ 130

Glossary 132